Olefin polymerizations such as ethylene polymerization are frequently carried out using monomer, diluent and catalyst and optionally co-monomers in a loop reactor. The polymerization is usually performed under slurry conditions, wherein the product consists usually of solid particles and is in suspension in a diluent. The slurry contents of the reactor are circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent, the product being often taken off via several settling legs which are mounted directly on the polymerization reactor and are periodically filled and discharged in a sequential manner to recover the product. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product slurry is further discharged to a flash tank, through flash lines, where most of the diluent and unreacted monomers are flashed off and recycled. Alternatively, the product slurry may be fed to a second loop reactor serially connected to the first loop reactor where a second polymer fraction may be produced. Typically, when two reactors in series are employed in this manner, the resultant polymer product, which comprises a first polymer fraction produced in the first reactor and a second polymer fraction produced in the second reactor, has a bimodal molecular weight distribution. These techniques have enjoyed international success with millions of tons of ethylene polymers being so produced annually.
Optimal behavior of the settling legs is reached when the quantity of recovered polymer is maximized with respect to the amount of fluid effluent that must be recycled, so that the recycling cost may be minimized for a given production rate. Classically, operation of the plant is based on attempting to discharge the same amount of slurry from all settling legs in order to afford equivalent pressure drops when discharging each leg, however this operation may be far from optimal.
Various alternative product removal techniques are however known. For example, WO 01/05842 describes an apparatus for removing concentrated slurry from a flowing stream of slurry in a conduit characterized by a channel in an outlet area of the conduit, the outlet being adapted to continuously remove slurry.
EP 0891990 describes an olefin polymerization process wherein the product slurry is recovered by means of a continuous product take off, more in particular by means of an elongated hollow appendage provided on the reactor. Said hollow appendage being in direct fluid communication with a heated flash line and thus being adapted for continuous removal of product slurry.
U.S. Pat. No. 4,613,484 describes a method and an apparatus for the separation of solid particulate polymer from liquid diluent comprising the use of one or more accumulator legs to which diluent is supplied, and separated diluent is recycled to the polymerization process.
EP 1 564 224 describes a set of loop reactor and a method to switch from series to parallel configuration for these loop reactors, using a three-or more-way valve positioned after the control valve of one or more settling legs.
However the above-described known apparatuses and processes have the disadvantage that the suspension withdrawn from the reactor still contains a large amount of diluent and other reactants, such as monomer, which implies the necessity of subsequently separate it from the polymer particles and to reprocess it for the purpose of reusing it in the reactor.
U.S. Pat. No. 6,586,537 describes a process wherein the product slurry is recovered by means of a hydrocyclone separator, which concentrates the slurry using centrifugal forces. This hydrocyclone is operated with a liquid stream nearly depleted in solids thereby necessitating a tight control of the operation conditions.
It is therefore an object of the present invention to provide a loop reactor having improved operating conditions. It is further an object of the invention to provide an improved polymerization process occurring in a loop reactor wherein the polymer slurry is efficiently removed from said reactor. Another object of the invention is to provide an improved process wherein the settling efficiencies of the polymer slurry and its further discharge are optimized. Another object of the present invention is to increase the reactor throughput by providing stable operation conditions. Another object of the present invention is to increase the weight percent (wt %) of polymer solids in the polymerization slurry circulating in the polymerization zone in the loop reactor.